Diffusion promoters for low temperature case hardening

ABSTRACT

Low temperature carburization of a workpiece surface is accomplished faster by impregnating the surface with a diffusion promoter prior to or during the low temperature carburization process.

RELATED APPLICATIONS

The present application claims the benefit of pending U.S. provisional application Ser. No. 60/921,935 filed on Apr. 5, 2007, for DIFFUSION PROMOTERS FOR LOW TEMPERATURE CASE HARDENING and U.S. provisional application Ser. No. 60/931,064 filed on May 21, 2007, for DIFFUSION PROMOTERS FOR LOW TEMPERATURE CASE HARDENING, the entire disclosures of which are fully incorporated herein by reference.

BACKGROUND

Case hardening is a widely used industrial process for enhancing the surface hardness of shaped metal articles. In a typical commercial process, the workpiece is contacted with a gaseous carbon compound at elevated temperature whereby carbon atoms liberated by decomposition of the carbon compound diffuse into the workpiece's surface. Hardening occurs through the reaction of these diffused carbon atoms with one or more metals in the workpiece thereby forming distinct chemical compounds, i.e. carbides, followed by precipitation of these carbides as discrete, extremely hard, crystalline particles in the metal forming the workpiece's surface. See, Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4, ASM Handbook, © 1991, ASM International.

Carbide precipitates not only enhance surface hardness, they can also promote corrosion. For this reason, stainless steel is rarely case hardened by conventional gas carburization, since the corrosion resistance of the steel is compromised.

In the mid 1980's, a technique for case hardening stainless steel was developed in which the workpiece is contacted with a carburizing gas at low temperature, typically below 500° C. (932° F.). At these temperatures, and provided that carburization does not last too long, carbon atoms diffuse into the workpiece surfaces, typically to a depth of 20-50μ, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained, which is believed due to the stress placed on the crystal lattice of the metal by the diffused carbon atoms. Moreover, because carbide precipitates are absent, the corrosion resistance of the steel is unimpaired, even improved.

This technique, which is referred to a “low temperature carburization,” is described in a number of publications including U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510, U.S. Pat. No. 5,792,282, U.S. Pat. No. 6,165,597, U.S. Pat. No. 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853). The disclosures of these documents are incorporated herein by reference.

SUMMARY

In accordance with this invention, it has been found that low temperature carburization of a workpiece surface can be accomplished faster by impregnating the surface with a diffusion promoter prior to or during the low temperature carburization process.

Thus, this invention in one embodiment provides a low temperature carburized stainless steel product having an altered surface layer which (a) contains an increased concentration of carbon atoms relative to the base metal from which the product is made, (b) is harder than the base metal from which the product is made, (c) is free of carbide precipitates, and (d) contains an increased concentration of at least one promoter element selected from Mo, Ni, Cr, Ti, V and Nb relative to the base metal from which the product is made.

In addition, this invention in a broader embodiment provides a metal product produced by subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of a diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element, wherein the altered surface layer further contains an increased concentration of at least one promoter element capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.

In addition, this invention further provides a process for enhancing the rate at which a diffusing element diffuses into the surface of a metal workpiece in a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of the diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element, the process comprising treating the workpiece to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.

Finally, this invention also provides a process for altering the physical properties of the surface layer of a metal workpiece by a low temperature diffusion-based surface treatment in which the metal workpiece is contacted with a gas containing a diffusing element at a treatment temperature which is high enough to cause the diffusing element to diffuse into the workpiece surfaces thereby producing an altered surface layer, the treatment temperature also being low enough to prevent formation of precipitates of compounds of the diffusing element, wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.

DETAILED DESCRIPTION

In accordance with this invention, low temperature carburization of a workpiece surface is accomplished faster by impregnating the surface with a diffusion promoter prior to or during the low temperature carburization process.

Low Temperature Carburization

The primary focus of this invention is on the low temperature carburization of iron-, nickel- and cobalt-based alloys, especially stainless steel. In this process, which is extensively described in the above-noted U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510, U.S. Pat. No. 5,792,282, U.S. Pat. No. 6,165,597, U.S. Pat. No. 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853), elemental carbon diffuses into the metal matrix forming the workpiece without formation of carbide precipitates.

Low temperature carburization normally produces an outermost oxide surface layer on the workpiece being treated about 20-30 nm thick. See, Japan 9-71853 (Kokai 9-71853). Depending on the carburizing conditions, this outermost oxide surface layer may also be covered with soot. In addition, under this oxide surface layer, an extremely thin outer surface layer of the metal may contain a small amount of carbide precipitates, especially if the low temperature carburization conditions are too severe. See, U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510 and U.S. Pat. No. 5,792,282. In order for the workpiece to exhibit an attractive metallic appearance, this soot and outermost oxide surface layer must be removed. In addition, this extremely thin outermost metal surface layer must also be removed in order for the workpiece to exhibit good corrosion resistance, at least if this outermost metal surface layer contains carbide precipitates. Therefore, as a practical matter, these unwanted by-products of the low temperature carburization process (i.e., the soot, oxide surface layer, and thin outermost metal layer containing carbide precipitates, if any) are removed before the workpiece is used. Accordingly, in the context of this disclosure, reference to a workpiece surface layer which is “free of carbide precipitates” or which is made “without formation of carbide precipitates” refers to the carbon-hardened surface layer remaining after the unwanted by-products of the low temperature carburization process (i.e., the soot, oxide surface layer, and thin outermost metal layer containing carbide precipitates, if any) are removed.

In this invention, low temperature carburization is carried out in the same way as done in the past by contacting the workpiece with a carburizing gas at an elevated temperature for a time sufficient to produce in the primary surface layer of the workpiece (i.e. the surface layer of the workpiece after the unwanted by-products of the low temperature carburization process are removed) an elevated amount of elemental carbon without formation of carbide precipitates. For this purpose, the carburization temperature will normally be no greater than about 500° C, although higher temperatures can be used by following the modified approach described in commonly assigned U.S. Pat. No. 6,547,888. Moreover, carburization will normally last 20-50 hours, although longer or shorter processing times can be used. As a result, a primary surface layer typically about 20-50μ thick and normally containing about 2-15 atomic %, more typically about 5-10 atomic % or even 9-12 atomic % atomic carbon will be obtained.

Incidentally, because low temperature carburization is a diffusion-based process, the concentration of carbon in the workpiece surfaces decreases from a maximum at or very near the outermost surface of the workpiece down to an equilibrium value (which is the carbon concentration in the “native” or untreated metal from which the workpiece is made) in accordance with Fick's law. Thus, it will be understood that the above reference to a carbon concentration of about 2-15 atomic %, for example, means that this is the carbon concentration at or near the outermost edge of the primary surface layer of the workpiece.

Other Low Temperature Diffusion-Based Surface Treatments

This invention concentrates on low temperature carburization of iron-, nickel- and cobalt-based alloys. In this context, an alloy which is “based” on a particular metal means that the alloy contains at least 35% of that metal. However, this invention is also applicable to other analogous low temperature diffusion-based surface treatments as well.

In low temperature carburization, as indicated above, atomic carbon diffuses interstitially into the workpiece surfaces, i.e., carbon atoms travel through the spaces between the metal atoms without significant substitutional diffusion of the metal atoms. Because the processing temperature is low, these carbon atoms form a solid solution with the metal atoms of the workpiece surfaces. They do not react with these metal atoms to form other compounds. Low temperature carburization is therefore different from normal carburization carried out at higher temperatures in which the carbon atoms react to form carbide precipitates, i.e., specific metal compounds such as M₂₃C₆ (e.g., Cr₂₃C₆ or chromium carbide), M₅C₂ and the like, arranged in the form of discrete phases separate and apart from the metal matrix in which they are contained.

Other analogous processes are known for altering the surface characteristics of a metal workpiece by interstitial diffusion of atoms into the workpiece surfaces at relatively low temperatures to form solid solutions with the metal atoms therein without formation of new compounds in separate phases. Examples include carburizing chromium- and nickel-based alloys, nitriding and carbo-nitriding of iron-, chromium- and/or nickel-based alloys, carburizing, nitriding and carbo-nitriding of titanium-based alloys, and infusing atomic nitrogen, carbon, boron or mixtures of these elements into aluminum and its alloys, to name a few. For convenience, all of these processes will be referred to collectively as “low temperature diffusion based surface treatments” or “low temperature” treatments.

The present invention is applicable to all such low temperature diffusion-based surface treatments. That is to say, the speed or rate at which each of these low temperature diffusion-based surface treatments can be carried out can be enhanced by adopting the principles of this invention. Thus, it will be appreciated that, although this invention is described here in terms of low temperature carburization for convenience, this invention also applies to such other analogous processes as well.

Enhancing Diffusion

In accordance with this invention, it has been recognized that the presence of certain alloying elements in a metal workpiece subjected to a low temperature diffusion-based surface treatment promotes or facilitates the speed or rate at which this surface treatment occurs. For example, it is very difficult to low temperature carburize Alloys 440C (S44004) and 410 (S41000), which are iron-based alloys containing 16-18% Cr and 12-13% Cr, respectively, and not much else, since the diffusion of carbon atoms into these alloys is very slow. In contrast, iron-based alloys M152 (S32550), AM355 \and 17-4 PH (S17400) carburize very easily, even though they contain essentially the same amount of Cr, i.e., 15-17%. However, Alloys M152 (S32550), AM355 and 17-4 PH (S17400) also contain 2-5% Ni, 2-3% Mo, or both, which indicates that these additional alloying elements, i.e., the Ni and Mo, promote the rate at which low temperature carburization of these alloys occurs. Cr, Ti, V and Nb have also been observed to impart a similar promoting effect on the ability of certain other iron-based alloys to respond to low temperature carburization. Accordingly, recognition of this phenomenon is taken advantage of in this invention by treating a workpiece made from such an alloy, prior to or during the low temperature carburization process, to cause one or more of these “promoter elements” to be taken up by and diffuse into the workpiece surface. Then, when the low temperature carburization is carried out, it can be completed faster (and/or a thicker hardened surface can be achieved in the same amount of time) than would otherwise occur, because the promoter element facilitates diffusion of carbon into the metal matrix of the workpiece surface.

The amount of promoter element that should be added to the workpiece surfaces to promote the low temperature diffusion-based surface treatment varies depending on the composition of the workpiece being treated and the particular promoter element chosen. For example, it has been found that the addition of about 0.5-5 wt. % Mo, Ni, Cr, Ti, V and/or Nb significantly enhances the rate with which elemental carbon is taken up by iron-based workpieces. However, greater or lesser amounts will still achieve some benefits and still can be used. For example, amounts as low as 0.1 wt. % and as high as 5 wt. % can be used. Additions on the order of 0.3 to 2.0 wt. % are more common. In this regard, routine experimentation can be used to determine the optimal amounts of particular promoter elements to use on particular alloys. Similar amounts of promoter elements are believed appropriate for non-iron-based alloys, it being understood that routine experimentation may be helpful for determining the appropriate amounts of particular promoter elements to use on these alloys as well.

As indicated above, the primary focus of this invention is on the low temperature carburization of iron-based alloys, especially stainless steels and the like. Alloys 440C (S44004) and 410 (S41000) mentioned above are particularly interesting. However, it is also applicable to other low temperature diffusion based processes as well.

So, for example, when carbon is diffused into a chromium-steel alloy, Ni and Mo are believed to be helpful in promoting the diffusion process.

Similarly, when carbon is diffused into a nickel-based alloy or a stainless steel alloy, Mo, Ti, V and Nb are believed to be helpful in promoting the diffusion process.

When nitrogen is diffused into a carbon or low alloy steel, Cr, Ni and Mo are believed to be helpful in promoting the diffusion process.

When carbon or nitrogen are diffused into a titanium-based alloy, V and Nb are believed to be helpful in promoting the diffusion process.

When carbon, nitrogen or boron are diffused into an aluminum-based alloy, Ti, V and Nb are believed to be helpful in promoting the diffusion process.

Such promoting elements can be used singly or in combination.

In the same way, it is also believed that when other low temperature diffusion-based surface treatments are carried out with other metal/diffusing element combinations, appropriate promoter elements will enhance the rate at which diffusion occurs. Thus, this invention can be used with respect to all such other combinations as well. That is to say, while all combinations of all promoter elements which will enhance the diffusion of a particular element into a particular metal in a low temperature diffusion-based processes are not known as of this writing, such combinations can be easily determined by routine experimentation. Therefore, even though these particular combination may not be known today, this invention is applicable to such combinations.

It should also be appreciated that there is an inherent limit to the diffusion-enhancing effect of promoter elements in general and that, as a result, little or no enhancement in the speed or rate of diffusion may be possible for certain alloys. So, for example, many stainless steels already contain significant amounts of Ni and Cr. Thus, the AISI 300 series steels such as AISI 301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless steels, for example, already contain significant quantities of Ni, Cr and, in some instances, Mo. Little if any improvement in the diffusion rate of carbon into these steels by low temperature carburization is achievable by this invention, relative to standard practice, because these steels already contain suitable quantities of promoting elements.

In contrast, the AISI 400 series stainless steels such as Alloy 410, Alloy 416 and Alloy 440C, which contain far less nickel, are difficult to process by standard low temperature carburization due to a low acceptance of carbon. These alloys can be processed with particular advantage according to this invention, since the presence of additional amounts of diffused nickel according to this invention will significantly enhance the rate at which diffusion of carbon into these alloys occurs. Similarly duplex steels, which are composed of ferritic regions rich in Cr but poor in Ni as well as austenitic regions with moderate Cr content but rich in Ni, can be processed with advantage by this invention, since the addition of a suitable diffusing agent such as Ni to the steel as a whole will allow its ferritic regions to carburize more easily. Alloys 2205 and 2507 are good examples of such duplex steels.

Applying the Promoting Elements

In accordance with this invention, the speed or rate at which a low temperature diffusion-based surface treatment of a metal workpiece can be completed (and/or the thickness of the altered surface layer obtained can be increased) by impregnating the surface with a diffusion promoter prior to or during the low temperature carburization process. This can be done by any process or technique which will increase the concentration of the promoter element in the surface of the workpiece which is to be altered by the low temperature diffusion-based surface treatment. In other words, this can be done by any process or technique which increases the concentration of the promoter element in the surface of the workpiece prior to the start of the low temperature diffusion-based surface treatment.

For example, the workpiece can be coated with a layer of the promoter element by any known coating technique and then heated to elevated temperature to drive the promoter element into the workpiece surface. For example, a workpiece made from an iron-based alloy can be provided with a coating of the promoter element by electroplating, electroless plating techniques, plasma coating, dipping in molten metal, painting with a paint containing the promoter element, or any other technique which will provide a layer of the promoter element in contact with the workpiece surfaces. Normally, the promoter element will be present in elemental form, although it can also be present in the form of a compound which decomposes to yield the promoter element at the conditions employed for driving the promoter element into the workpiece surfaces. Thereafter, the coated workpiece can be heated to elevated temperature, e.g., 1,000° C. (1832° F.) or more, to drive the promoter element from the coating into the workpiece. A modification of this technique based on ALD (atomic layer deposition) in which the workpiece is subjected to repeated iterations of deposition and diffusion can also be used.

Other techniques capable of causing promoter elements to diffuse into metal surfaces can also be used. For example, gas phase diffusion processes analogous to the low temperature diffusion-based process described above can be used, i.e., diffusion-based processes in which the promoter element, or a compound capable of decomposing to yield this element, is contacted with the workpiece surface in the form of a gas. Alternatively, the so-called GE Metalliting process in which the workpiece is contacted with a molten bath of a fluoride salt containing the promoter element can be used.

In still another approach, a workpiece previously treated to make its surface porous can be impregnated with the promoter element, or a compound capable of decomposing to yield this element, and then heated to elevated temperature to promote diffusion of this element into its surfaces. Examples of techniques that can be used for making the workpiece surface porous include contact with hot, concentrated HCl, aqua regia or the like, mechanical abrasion, anodizing the workpiece, and growing a porous oxide layer the workpiece surface in the manner described in the Background Section above (i.e., by exposing the surface to oxygen or a compound capable of liberating oxygen under the conditions encountered by the workpiece).

Most often, impregnation of the workpiece with the promoter element will be done prior to the start of the low temperature diffusion based surface treatment. However, impregnation can also be done during the low temperature diffusion based surface treatment, for example, by interrupting (i.e. stopping) this surface treatment, applying the promoter element in the manner described above during this interruption, and then resuming the low temperature diffusion based surface treatment. In any event, regardless of the particular technique employed, the workpiece is impregnated with a promoter element such that the altered surface layer in the metal product ultimately produced by the low temperature diffusion based surface treatment, in addition to containing an increased concentration of the diffusing element responsible for altering the properties of this surface layer, further contains an increased concentration of at least one promoter element capable of enhancing the rate at which this diffusing element diffuses into the workpiece surface layer.

In this connection, it has already been proposed to activate a stainless steel workpiece for subsequent low temperature carburization by providing the workpiece with an electroless nickel coating. See, U.S. Published Patent Application U.S. 2006/0090817 A1. When such a workpiece is low temperature carburized, it is believed that some of the nickel in this coating diffuses into the workpiece surface as a by-product of the low temperature carburization process. That is to say, it is believed that the conditions of time and temperature involved in low temperature carburization are severe enough so that some incidental amount of nickel atoms in this coating diffuse from atop the workpiece surface into this workpiece surfaces along with the carbon atoms that diffuse into this surface during the low temperature carburization step. The invention described here differs from that prior practice in that, in this invention, more than incidental diffusion of nickel or other promoter elements is involved. In other words, in this invention the amount of promoter element which diffuses into the workpiece surfaces is greater than the incidental amount of nickel atoms that may have diffused into the workpiece surfaces in that earlier technology. Therefore, it will be appreciated that reference in this document to an “increased concentration of promoter element” in the surface layer of the metal product produced by this invention means an increase over and above the incidental increase that might occur in that earlier technology as a by-product of the low temperature diffusion based surface treatment.

Activation

Some metals form coherent protective oxide coating layers essentially instantaneously upon contact with air. A good example is aluminum and its alloys, which form coherent protective coating layers of aluminum oxide. Another example is stainless steel, which forms a coherent protective coating layer of chromium oxide. These protective coherent oxide coating layers are impervious to most materials, including the diffusing elements typically used in most low temperature diffusion-based processes. Accordingly, these workpieces are typically “activated” before or simultaneously with the diffusion-based surface treatment to make this coating permeable to the diffusing element being used.

Activation is normally done by treatment with a halogen-containing gas such as F₂, Cl₂, HCl, HF, NF₃ and the like. Treatment with acids in liquid form, particularly aqueous compositions of strong acids, such as aqueous HCl, H₂SO₄, HNO₃, aqua regia and the like can also be used. Activating can be also be done mechanically, for example, by sawing, scraping or sanding the workpiece to expose the “native” metal of the surface being treated. In addition, activating can also be done electrochemically, i.e., by contacting the metal with an electrolyte and subjecting the metal to a sufficient electric potential to cause anodic decomposition.

When the workpiece being treated by this invention is made from a metal which does not form such a coherent protective oxide coating, no special pretreatment or other procedures is needed for causing the selected promoter element to diffuse into the workpiece surfaces. However, where this metal does form a coherent protective oxide coating, some form of activation may be necessary depending on the approach used for supplying the promoter element and/or driving the diffusing element into the workpiece surfaces.

In this connection, many of the approaches described above for causing promoter elements to diffuse into a workpiece's surface inherently depassivate any coherent protective oxide layer that might be present at the same time. For example, heating a workpiece to elevated temperature, e.g., 1,000° C. (1832° F.) or more, for driving a promoter element into the workpiece's surface will also depassivate the protective, coherent protective oxide coating formed by most metals. Similarly, electrolysis metal coating techniques will also depassivate the protective, coherent oxide coating formed by most metals. Similarly, the GE Metalliting process in which the workpiece is contacted with a molten bath of a fluoride salt containing the promoter element will also depassivate most protective, coherent oxide coatings. So in many instances, no special activation or pretreatment is required for practicing this invention.

However, in those instances where the protective, coherent oxide coating is not depassivated by the particular promoter-diffusion approach used, the workpiece can be pretreated to depassivate this oxide coating before the promoter element is applied, or at least before the workpiece is treated to drive a previously-applied promoter element into its surfaces.

This pretreatment can be done in the same way as described above for activating the workpiece in traditional low temperature diffusion based surface treatments such as, for example, by contact with a halogen-containing gas or strong acid, by mechanically exposing the workpiece's native metal, or electrochemically. Once this is done, the workpiece can be subjected to the particular coating/diffusion approach selected for diffusing the promoter element into the workpiece surface. Normally, this will be done without exposing the activated workpiece to the atmosphere to avoid repassivating the previously-depassivated surfaces. However, it is normally not necessary to keep the workpiece out of contact with the atmosphere in those situations in which the particular coating/diffusion approach selected provides its own coherent impervious coating of the promoter element, or a compound capable of decomposing to yield this element. This is because this promoter element coating will normally prevent repassivation from occurring.

Product Workpiece

As indicated above, the metal products produced by earlier low temperature diffusion-based processes include an altered surface layer containing an increased concentration, relative to the “native” metal from which the workpiece is made, of a diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer. Nonetheless, this altered surface layer is still free of precipitates of compounds formed from the diffusing element. Thus, for example, when a stainless steel workpiece is low temperature carburized, an altered surface layer is obtained which is not only free of chromium carbide precipitates but, in addition, exhibits greater hardness due to the presence of the diffused carbon atoms.

This same result is also achieved in this invention. However, this invention departs from earlier work in that, in this invention, the surface of the workpiece which is subjected to the low temperature diffusion-based process further includes an increased concentration of at least one promoter element capable of enhancing the rate at which the diffusing element diffuses into the workpiece's surfaces. Moreover, the amount of this increase is greater than any incidental increase that may have occurred as a by-product of the low temperature carburization process described in U.S. 2006/0090817 A in which the workpiece is activated by means of an electroless nickel coating. The result is that the low temperature diffusion based process can be completed faster, or a deeper altered surface layer can be produced in the same amount of time, relative to traditional practice.

Although only a few embodiments of this technology have been described above, it should be appreciated that many modifications can be made. All such modifications are intended to be included within the scope of this disclosure, which is to be limited only by the following claims. 

1. A metal product produced by subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of a diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element, wherein the altered surface layer further contains an increased concentration of at least one promoter element capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.
 2. The metal product of claim 1, wherein the metal workpiece is made from a carbon or low alloy steel, a nickel-based alloy, a chromium-steel alloy, a titanium based alloy or an aluminum-based alloy.
 3. The metal product of claim 2, wherein the altered surface layer is produced by the low temperature carburization of a stainless steel workpiece to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates.
 4. The metal product of claim 2, wherein the altered surface layer is produced by carburizing, nitriding or carbo-nitriding a carbon or low alloy steel, a chromium-steel alloy, stainless steel, or a nickel-based alloy.
 5. The metal product of claim 2, wherein the altered surface layer is produced by carburizing, nitriding or carbo-nitriding a titanium-based alloy.
 6. The metal product of claim 2, wherein the altered surface layer is produced by infusing atomic nitrogen, carbon, boron or mixtures of these elements into an aluminum-based alloy.
 7. A low temperature carburized stainless steel product having an altered surface layer which (a) contains an increased concentration of carbon atoms relative to the base metal from which the product is made, (b) is harder than the base metal from which the product is made, (c) is free of carbide precipitates, and (d) contains an increased concentration of at least one promoter element selected from Mo, Ni, Cr, Ti, V and Nb relative to the base metal from which the product is made.
 8. The process of claim 7, wherein the stainless steel product is made from an AISI 400 series stainless steel.
 9. A process for enhancing the rate at which a diffusing element diffuses into the surface of a metal workpiece in a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of the diffusing element in an amount sufficient to alter the properties of the metal forming the surface layer, the altered surface layer also being free of precipitates of compounds of the diffusing element, the process comprising treating the workpiece to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.
 10. The process of claim 9, wherein the concentration of the promoter element in the surfaces to be altered is increased (a) by applying a coating containing the promoter element, or a compound capable of decomposing to yield the promoter element, to these surfaces and then heating the applied coating to drive the diffusing element into these surfaces, or (b) by a gas phase diffusion process in which the promoter element, or a compound capable of decomposing to yield this element, is contacted with the workpiece surface in the form of a gas, or (c) by contacting the workpiece with a molten bath of a fluoride salt containing the promoter element, or (d) by treating the surfaces to be altered to make them porous, impregnating these porous surfaces with the promoter element, or a compound capable of decomposing to yield this element, and then heating these impregnated surfaces to elevated temperature to promote diffusion of the promoter element into these surfaces.
 11. The process of claim 9, wherein the metal workpiece is made from a carbon or low alloy steel, a nickel-based alloy, a chromium-steel alloy, a titanium based alloy or an aluminum-based alloy.
 12. A process for altering the physical properties of the surface layer of a metal workpiece by a low temperature diffusion-based surface treatment in which the metal workpiece is contacted with a gas containing a diffusing element at a treatment temperature which is high enough to cause the diffusing element to diffuse into the workpiece surfaces thereby producing an altered surface layer, the treatment temperature also being low enough to prevent formation of precipitates of compounds of the diffusing element, wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered, the promoter element being capable of enhancing the rate at which the diffusing element diffuses into the surface of the workpiece to be altered during the low temperature diffusion-based surface treatment.
 13. The process of claim 12, wherein the low temperature diffusion-based surface treatment comprises carburizing, nitriding or carbo-nitriding a titanium-based alloy.
 14. The process of claim 12, wherein the low temperature diffusion-based surface treatment comprises infusing atomic nitrogen, carbon, boron or mixtures of these elements into an aluminum-based alloy.
 15. The process of claim 12, wherein the low temperature diffusion-based surface treatment comprises carburizing, nitriding or carbo-nitriding an iron-based alloy, a chromium-based alloy or a nickel-based alloy.
 16. The process of claim 15, wherein a stainless steel workpiece is low temperature carburized to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates.
 17. The process of claim 12, wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered prior to the start of the low temperature diffusion based surface treatment.
 18. The process of claim 12, wherein the workpiece is treated to increase the concentration of at least one promoter element in the surface to be altered during the low temperature diffusion based surface treatment by interrupting this surface treatment, treating the workpiece to increase the concentration of at least one promoter element in the surface to be altered during this interruption, and thereafter resuming the low temperature diffusion based surface treatment. 